The present invention generally relates to a method of reinforcing an orthopedic implant and to an implant product. More particularly, the present invention is directed to an improved method of implanting a prosthetic device for counteracting the stresses and strains to which a prosthetic device is typically exposed.
Prosthetic devices are artificial devices used to replace or strengthen a particular part of the body. Such devices can be used in humans or animals to repair or replace diseased or damaged bone, allied tissue associated with the bone, and/or joints associated with the bone. In very general terms, a prosthesis is used to correct or prevent skeletal deformities and to alleviate pain and discomfort caused thereby.
Currently, the most common way to implant a prosthesis is to first prepare a receiving site or cavity in an adjoining bone. A bone cement is placed in the receiving site. A prosthesis is then positioned in the bone cement, while the cement is cured or polymerized.
In most applications, an acrylic bone cement is used. Typically, the bone cement includes an acrylic polymeric powder, such as polymethyl methacrylate (PMMA). The acrylic polymeric powder is premixed with a liquid acrylic monomer system, which may include methyl methacrylate (MMA), resulting in a substance with a dough-like consistency, which is subsequently applied to a bone cavity. After being placed within the cavity, the bone cement is then cured or polymerized and hardened in order to secure the prosthesis within the bone.
Once implanted, a prosthetic device ideally closely assimilates the characteristics of the bone and/or the joint that the device is intended to repair or replace. Further, the implanted prosthetic device should be capable of supporting and withstanding stresses and strains normally imparted to the repaired or replaced bone.
Although the above process for implanting prosthetic devices is generally accepted within the art and has proven to be a successful process for repairing or replacing damaged bones and the like, various problems were still encountered in the past. For example, prosthetic devices were prone to loosen within the bone cavity over time. In particular, most bone cements are neither as strong nor as flexible as bone tissue. Consequently, the bone cement can break away from the prosthesis, can break away from the bone, or can develop stress or fatigue cracks when repeatedly exposed to the normal stress and strains supported by bones.
Due to these problems, attempts have been made to improve the mechanical properties of prosthetic devices and of the cement interface that exists between the device and the bone. For instance, U.S. Pat. No. 4,491,987, filed by the current inventor, which is incorporated herein in its entirety by reference, discloses an improved prosthesis and process for orthopedic implantation of the prosthesis. In that application, a prosthesis is precoated with a polymeric material that is compatible with bone cement. Once implanted, the precoat provides a stronger interfacial bond between the bone cement and the prosthesis.
In U.S. Pat. No. 4,735,625 to Davidson, a prosthesis for reinforced bone cement implantation is disclosed. In one embodiment, the bone cement is reinforced by a mantle or sock formed from a plurality of biocompatible oriented fibers, e.g., polyethylene, carbon, stainless steel or the like. The mantle or sock is sized and shaped to fit over the stem of a typical hip prosthesis. The mantle or sock can be used to form part of a composite precoat by being bonded to the prosthesis prior to implantation.
In another embodiment in Davidson, the fibers can be in the shape of a rectangular mat or in a general cylindrical mantle and embedded in the bone cement. Specifically, it is taught to place the mat or mantle adjacent with the upper portion or proximal end of the stem of the prosthesis.
In U.S. Pat. No. 5,035,714 to Willert, et al., a reinforcement for a bone cement bed is disclosed. The reinforcement includes a grid of crossing members at least some of which are wavy. The reinforcement is provided between the prosthesis and the bone for positioning the prosthesis in place as well as for strengthening the bone cement bed. In particular, the reinforcement is placed at the proximal end of the prosthesis, similar to the embodiment disclosed in Davidson.
The prior art has focused almost exclusively on increasing the fatigue and static fracture resistance of the bone cement bed and on increasing the strength of the interfacial bond between the prosthesis and the bone cement. The present application is directed to further improvements in a bone cement bed and prosthesis combination. Specifically, the present invention is directed to providing a method and implant product that is designed to withstand the radial-stresses and hoop-stresses normally associated with a prosthetic device. As used herein, hoop-stress refers to any circumferential stress or, in other words, any tangential stress that occurs at the periphery of the bone cement bed, at the bone wall, and/or at the outside surface of the prosthetic device.
In particular, hoop-stress can be created when the prosthesis is loaded. More specifically, hoop-stress results between the prosthesis and the surrounding bone cement bed when forces are exerted on or near the end of the prosthesis not embedded within the cement. Hoop-stress can also develop in the bone cement due to shrinkage of the bone cement during polymerization. Conventional bone cement deforms over time, which in turn causes loosening between the prosthetic device and the bone cement bed.
When uncontrolled, hoop-stress can cause the development of microcracks in the bone cement. Further, hoop-stress can cause the prosthesis to separate from the bone cement bed. As such, a need exists for an improved implant product and for a method of implanting a prosthesis that counteracts hoop-stress, decreases shrinkage creep of the bone cement, and further reinforces the bone cement bed.